1. Field of the Invention
This invention relates in general to a method of venting a furnace. More particularly, the invention relates to an improved method of venting a furnace which allows for field modification of the furnace depending on the type of installation required.
2. Discussion of the Related Art
In conventional gas-fired forced air furnaces a thermostat senses the temperature in the comfort zone relative to a predetermined set point temperature. When the temperature is below the set point, the thermostat closes to supply thermostat ac power to the furnace as a call for heat. This initiates a sequence of events that ultimately causes the furnace to come on. An inducer motor is enabled to flow air through the heat exchangers for combustion, after which a gas valve is actuated to supply gas to the gas burners. An ignition device is also actuated to light the burners. In some furnace designs, a flame sensor then proves burner ignition. Then, after a predetermined blower delay time, which varies with furnace design, the furnace blower is actuated. The blower circulates room air from the return air duct over the furnace heat exchangers to pick up heat from the hot combustion products (carbon dioxide, nitrogen, oxygen, excess air and water vapor). The heated circulating air then goes into the supply air plenum and is distributed by ductwork back to the living space. When the living space is warmed sufficiently to reach the thermostat set point, the thermostat terminates the call for heat. When this happens, the blower and burners go through a shut off sequence and the furnace awaits the next call for heat.
After passing through the heat exchanger, the combustion byproducts are vented outside of the structure through a vent pipe. The vent pipe can be oriented either predominantly horizontally through a side wall of the structure or predominantly vertically through the roof of the structure. When the inducer motor is in operation, a substantial step-up in pressure occurs between the intake of the inducer housing (the collector box) on the one hand, and the outflow of the inducer housing (the relief box) on the other hand. Typically there is negative pressure (relative to atmospheric pressure) at the intake. The pressure at the outlet of the inducer housing is slightly negative for conventional vertical vent systems, and substantially positive for horizontal side vent systems.
One problem with furnaces in the past is that with a horizontally vented furnace, the furnace is affected by wind conditions such that under certain outside conditions, such as high wind conditions, back pressure can cause the inducer to become overloaded. However, a vertically vented furnace is not affected as much by wind conditions because of the buoyancy of the heated air and the angle of incidence of wind on the vent termination. In order to minimize the decrease in pressure drop across the inducer caused by wind in a horizontally vented furnace and improve wind resistance, the pressure drop across the inducer must be great enough to offset the back pressure.
A second problem with furnaces of the past relates to condensation in the chimney. In a vertically vented furnace, based on geographic location, furnace input, and liner interior dimension, the chimney can either require relining, if it is inadequately lined or be adequately lined. The lining can be a tile liner or a pipe inserted into the chimney. Unlined chimneys are more susceptible to damage (i.e. spalling and cracking when exposed to freezing conditions) caused by condensation in the chimney. Condensation is more likely to form in the chimney with more efficient contemporary furnaces. This occurs because the flue gasses are relatively cooler in a higher efficiency furnace than in a lower efficiency furnace and because the amount of excess air in higher efficiency furnaces is reduced. The cooler, reduced mass flow of flue gasses is more likely to condense water vapor on chimney walls and is less able to dry the walls during operation. In a lower efficiency furnace, less heat is transferred to the room air, causing the flue gasses to be warmer. Past lower efficiency designs also had greater amounts of excess air in the vent system. The warmer, higher mass flow of flue gasses removes more moisture from the chimney walls. Thus, in the past, when installing a high efficiency furnace with an unlined or inadequately lined chimney, a liner would have to be installed in the chimney, increasing the cost of the installation. Prior to the time the furnace is installed, it is not known whether the chimney will be lined or unlined.